A. Field of the Invention
The present invention relates to a method and apparatus to synchronize two or more clocks. More specifically, the present invention relates to a method of controlling a slave time by adjusting the slave time based on a master time. In one embodiment, a global positioning system (GPS) receiver calculates GPS time and sends a correction signal to a pseudo-satellite, or pseudolite (PL), in order to slave the PL time to GPS time.
B. Problems in the Art
The use of GPS signals to calculate position and GPS time is known in the art. There are times when it is difficult or impossible to receive the true GPS signals from the satellites. For example, there are times when the GPS signals are jammed in a localized area, or a GPS receiver may be located in a valley where it cannot receive signals from a sufficient number of satellites.
In order to make GPS signals available to receivers in locations where the true signal is being jammed, the concept of pseudolites was invented. A PL includes a PL transmitter that sends out pseudolite signals which are designed to simulate the true GPS signals from the satellites. An example of a ground-based PL is disclosed in U.S. Pat. No. 5,886,665. An example of an airborne pseudolite is disclosed in U.S. Pat. No. 5,886,666. The disclosures of U.S. Pat. Nos. 5,886,665 and 5,886,666 are hereby incorporated by reference in their entirety. A PL allows for reception of GPS signals in those locations where the true GPS signals are being jammed.
In order for a receiver to accurately determine a position solution from the PL signals, it is important that the PL signals precisely mimic the true GPS signals. In order for PL signals to accurately match true GPS signals, it is important that the PL time match GPS time. Relatively small time errors between the PL time and GPS time can introduce significant errors along the line of sight of the PL, thereby limiting navigational accuracy. In order to approximate GPS time, a PL could be provided with a high accuracy, costly clock, such as the clocks on the GPS satellites. Once the high accuracy clock was set to GPS time, variances from GPS time would not be as great. However, the high accuracy clock would still require a time synch to simulate GPS time. There is a need in the art for a method that provides precise time synchronization between GPS time and pseudolite time, which does not require the expense of a high accuracy clock.
Therefore, it is a primary object of the present invention to provide a method of time synchronization which solves problems and deficiencies in the art.
It is a further object of the present invention to provide a method which synchronizes pseudolite time with GPS time.
It is a further object of the present invention to make position solutions obtained from pseudolite signals more accurate.
It is a further object of the present invention to correct for environmental effects which limit the accuracy of a pseudolite clock.
It is a further object of the present invention to allow for the use of a less expensive, less accurate pseudolite clock, while still maintaining accurate time within the pseudolite.
These, as well as other objects and features of the present invention, will be apparent from the following detailed description and claims in conjunction with the accompanying drawings.
The present invention includes a method of synchronizing a slave time with a master time. The first step in the method is receiving a signal from a remote source from which the master time can be derived. After the signal is received, the slave time is compared to a master time to detect any offset of the slave time relative to the master time. If there is any offset, the slave time is adjusted to correct the offset.
The present invention can also include a method of synchronizing a time of a pseudolite transmitter with a time of a global navigational satellite system. The first step is initializing the pseudolite transmitter time. After the pseudolite transmitter time has been initialized, the time of the pseudolite transmitter is compared to global navigation satellite system (GNSS) time to determine any offset of the pseudolite transmitter time relative to GNSS time. If there is any offset, the pseudolite transmitter time is adjusted to correct the offset.
The present invention can also include an improved pseudolite. The improved pseudolite includes a global navigational satellite system (GNSS) receiver and a pseudolite signal transmitter. The receiver is in operative communication with the transmitter. The receiver is capable of receiving a signal from the transmitter from which the transmitter time can be derived. The receiver is programmed to compare the pseudolite time to a GNSS time to detect any offset of the pseudolite time relative to GNSS time, and to communicate a control signal to the transmitter to adjust the pseudolite time to correct the offset if an offset is detected.